As George Bush plays chicken with the Chinese and fast and loose with the environment, it is time for the left to play truth or consequence with Ralph Nader. The Green party presidential candidate stood against the two main parties, arguing that there was no difference between them and that America needed an alternative. America ended up with what looks like being the most rightwing president since before the war.

Bush didn't win the election, he won a court case. But, with the slenderest of endorsements, he promises to inflict severe damage. In Congress, many Republicans are beginning to think he is beyond the pale. Even Uncle Sam's faithful poodle, the British government, is yapping at his heels. Some in the cabinet are calling on Tony Blair to put the special relationship "into deep freeze". John Prescott flies to New York today to try to persuade Bush to change his mind about scrapping the Kyoto agreement on controlling greenhouse gases.

So first, some truth. There is a difference between Bush and his Democratic challenger, Al Gore. For Nader to have claimed otherwise during his campaign was disingenuous and opportunistic. We do not know what Gore would have done by this stage had he won, but we can be quite sure it would not have been this. His first decision as president would not have been to deny aid to non-governmental organisations that support abortion overseas through surgery, counselling or lobbying. Nor would Gore have put forward a budget planning to eliminate $309m in grants to help public housing authorities get rid of drug dealers or a programme to preserve wetlands so that he could give trillions to the wealthy. And least of all would he have turned his back on the Kyoto accord.

Now, for a consequence. If Nader had not stood, then Gore would be president. This is as close to a political fact as a statistic dares to be. True, polls showed that one-third of those who voted for Nader would otherwise not have voted at all. But it is also true that more than half of Nader's supporters would have voted for Gore, delivering him majorities in both Florida and New Hampshire and winning him the electoral college.

For the left not to acknowledge this is spineless. If it wants to be taken seriously it must first take itself seriously. Nader stood to make a difference and he succeeded. In politics, as in life, a sign of maturity is accepting responsibility for your actions. Moreover, only once those points have been conceded is it possible to mount a credible defence of Nader's candidacy. Because Nader not only had a right to stand but was right to stand. The problem with George Bush is not that he is a vicious rightwing ideologue - the man can barely tie his shoelaces - it is that he is the paid representative of corporate America.

It is no good challenging Bush without challenging the system that produced him - a system in which big money, not ideas, selects the candidates and then backs both sides to make sure it picks the winner. Since Gore and the Democrats were not only complicit in that system but abused it to their own ends while in office, they were incapable of taking on that task even if they had wanted to. It took an outsider. Nader alone provided a meaningful choice in what is rapidly becoming a multimillion-dollar, corporate-sponsored charade, masquerading as democracy.

Nader was right not because there was no difference between the two main parties but because there was insufficient difference. The Democrats' pitch to potential Nader supporters was: "At least we're not Republicans." The 2.7m people who voted for Nader felt they wanted more from democracy than that.

Democrats love to blame Nader for Bush. Their logic is sloppy. Democrats deny a myriad of other far more compelling or equally tenuous factors that put Bush in the White House. They ignore the fact that, after two terms in office, they could not win Clinton's or Gore's home states. They deny that the situation was so close in Florida that any candidate who stood, including the Natural Law Party, could reasonably claim to have made the decisive difference.

One could as well blame Theresa LePore, the election supervisor who botched the ballot papers, or Katherine Harris, the Republican secretary of state in Florida who obstructed the recounts. In such a tight race, Nader was a factor not the factor. Under such circumstances, to fixate on him as the principal reason for Gore's defeat is perverse.

The charge also reveals astonishing political arrogance. It suggests that the Democrats have a right to the left vote regardless of what they say and do. Clinton can withdraw welfare benefits from the poor, promote a free-trade agreement (Nafta) that sells jobs to the lowest wages and weakest unions in the continent, broker global trade deals that hammer the poor or starve Iraqi children and still expect liberals to turn out for his successor.

Democrats scream betrayal without realising that before there is betrayal there must first be friendship and trust. They demand loyalty, but show none in return. Having spent a decade distancing themselves from the left, they express shock that the left might choose to respect that distance and go it alone.

None the less, the question of whether a principled stand against big money is best served by the practical outcome of a Bush presidency remains pertinent. The answer may change as his term progresses. For the time being, on balance, it was. For evidence look no further than Kyoto. Bush's decision to renege on the treaty is a vicious attack on the environment. But Clinton's record was not much better. He signed up to Kyoto, but he did not honour it. The US, by far the world's largest polluter, promised to cut carbon-dioxide emissions by 7% from 1990 levels by 2012. Instead, emissions rose by more than 10% on 1990 levels by 2000.

It was thanks to Clinton's administration that last year's climate talks in the Hague collapsed. The problem was not only that he could not get the legislation through a Republican Congress, it was that he dared not take on the might of the oil and gas companies. They gave Republican candidates $10m last year; but they gave Democrats, including Gore, $4m. Bush may be in hock to them, but Clinton was in awe of them.

But much also depends on what Nader does now. The corporate domination of American politics cannot be undermined once every four years at election time or on television-panel discussions and the lecture circuit. The truth is that it will take not just a party but a movement, joining together the disparate voices of labour unions, tree huggers and pressure groups that made themselves heard at Seattle, to make complete sense of his candidacy. Having made a difference at the polls, he must now make a difference in civil society. Only then will it be clear that the consequence of Nader's candidacy was not to derail the Democrats, but to restore democracy.